Nothing is known about Black Sperm's past or how he came to be.
Black Sperm is first seen present at Gyoro Gyoro's speech at the Monster Association Headquarters. He is one of the monsters who speaks up about serving under Orochi, stating that they are only allies at the moment to crush the Hero Association. He backs off when Gyoro Gyoro reminds the monsters that anyone who can challenge and defeat Orochi can take his place.
Black Sperm also watches as Gyoro Gyoro offers Garou a position as an executive member of the Monster Association. He shows surprise at this and sarcastically quips at Gyoro Gyoro for giving Garou such a lucrative offer. Later, after Orochi and Garou's battle, Gyoro Gyoro calls all the Monster Association executives into the chamber, including Black Sperm. He tells them that he's "figured it out", telling them which S-Class hero each executive should challenge. Black Sperm is also later seen feeling the impact from Saitama's punch on Overgrown Rover.
Black Sperm is next seen chasing Tareo through the tunnels of the Monster Association base, toying with him and telling him he's going to kill him. He is interrupted by Atomic Samurai, whose slash he manages to avoid. Atomic Samurai is surprised by his speed, but notes that he's just a monster. Black Sperm is enraged, sensing that Atomic Samurai is looking down on him, and attacks him. Atomic Samurai slashes through him hundreds of times, but each time he only splits into more Black Sperm cells. There are eventually so many Black Sperms that Atomic Samurai is hopelessly surrounded, and Black Sperm tells him that this place will be his deathbed. Black Sperm proceeds to completely brutalize Atomic Samurai, outnumbering him and surprising him with his physical strength.
Black Sperm's beatdown of Atomic Samurai is interrupted by Tatsumaki, who creates a barrier around the hero and begins pulling him upwards as she prepares to make her move. Black Sperm is confused and frustrated by this, and grumbles as Atomic Samurai manages to get one last slash on him before floating away.
Black Sperm is pulled with the rest of the Monster Association headquarters above ground by Tatsumaki's insane psychic powers. While she deals with Black Sperm and the other executives, she is caught by surprise by Psykos, who hits her in the head hard enough to temporarily disable her powers. Black Sperm grabs Tatsumaki, but is attacked by Genos and drops her. Bang appears and disposes of Fuhrer Ugly and Gums, along with many of Black Sperm's cells, but is in turn incapacitated by Psykos and Homeless Emperor. Black Sperm also gets a surprise attack on Bomb while he is speaking with Genos about what to do. Genos then begins to fight the hordes of Black Sperm cells, but ultimately has his limbs torn off and is left on the ground.
Once King arrives, Black Sperm and the other executives focus their attention on him. Black Sperm is excited to fight "the world's strongest man" - however, he and the executives are caught off guard by how King doesn't attack. Homeless Emperor almost attacks first, but is hesitant once King points out he needs to watch his footing so he does not fall through the rubble beneath them. Black Sperm grows impatient with Homeless Emperor and notices that King has folded his arms, which he takes as a challenge, but he is interrupted by Fubuki's appearance. While Psykos deals with Fubuki, Zombieman appears and gets a surprise attack on Homeless Emperor, leaving Black Sperm alone with Evil Natural Water to face King. He is frustrated that Evil Natural Water isn't attacking yet, and deduces that King's lack of attacking is because he's looking down on them. Black Sperm grows angry and increases his cell count to 11,449,001,712,554 as he prepares to attack. He is interrupted again when Puri-Puri Prisoner emerges from the rubble with several heroes. As more and more heroes begin to appear, Sweet Mask throws a massive boulder at Black Sperm, who catches it with one hand. Black Sperm then begins to fight Sweet Mask.
Eventually, Tatsumaki regains her strength and retaliates, blowing away many of Black Sperm's cells with psychic wind. Black Sperm grows frustrated and combines 1,293,142,261,072 of his many cells into a powerful Multi-Cell Sperm. The other 10 trillion cells watch as the Multi-Cell Sperm attacks Tatsumaki, though he is quickly killed when his head is twisted off. The other cells are shocked, and Child Emperor points out that the merged Multi-Cell Sperm couldn't split like the single cells could, much to Black Sperm's annoyance. He sends 10,000 of his cells at Tatsumaki as a distraction, while he sends 100 merged cells at Tareo to hold him hostage.
While the 100 cells hold Tareo, the remaining 10 trillion cells combine into one superpowered body called Golden Sperm. Tatsumaki is unimpressed, but Golden Sperm surprises her with his lightning speed and quickly beats her with multiple direct blows. The 100 cell Black Sperm is amazed by Golden Sperm's strength, who tells him that he might as well absorb him too, but Black Sperm refuses. Golden Sperm is then attacked by Tatsumaki again, trying to break his neck with her psychic powers to no avail.
King suddenly reappears, calling out his "Hellfire Burst Wave Motion Cannon", and catches Black Sperm by surprise. However, the attack never comes, much to his relief, and he watches in confusion as King turns and runs away. He discusses chasing him with Golden Sperm, but Golden Sperm tells him they need to stay with the hostage, Tareo. The 100 cell body doesn't respond, having been cut in half as Garou emerges from the other side. Golden Sperm is surprised that Garou is still alive, and receives a brutal blow to the face. Black Sperm's remaining cells attack Garou, though he quickly wipes them out in an instant. The single remaining cell considers hiding, while Golden Sperm grows excited and moves to challenge Garou. He engages Garou, the two moving at blistering speeds that the heroes' eyes can't even keep up with. When the dust settles Golden Sperm lays dead on the ground, with Garou victorious.
The next day it is revealed that the single Black Sperm cell survived the battle. He emerges as Pig God is looking for the last of Evil Natural Water so he can finish it off, but stays hidden. Once Pig God departs, Black Sperm begins to make plans to regain his cell stock, but sees Saitama searching through the rubble for the remains of his apartment. Black Sperm is surprised to notice that the weakened Overgrown Rover is docile with Saitama, and realizes it might actually be safe to stay with such a powerful individual. Black Sperm then follows Saitama home, much to his chagrin.
Once they reach Saitama's new apartment, the security robots made by Metal Knight detect Black Sperm and Rover and attack. Saitama quickly deals with the robots, not realizing his followers are both technically monsters, and Black Sperm sneaks in with him by clinging to Rover's side, now realizing that he's entered the Hero Association's private apartment complex.
Saitama does not want to have to take care of Black Sperm and Rover, so he makes a bet with fellow hero Forte to make him keep them, which Saitama wins. The two formerly dangerous monsters are then entrusted to Butterfly DX and Chain'n'toad, living in a dog house on their apartment balcony.
While Butterfly DX, Chain'n'toad, and Forte discuss the Neo Heroes, Black Sperm begins to wonder if the Monster Association really did succeed in destroying the Hero Association after all, smirking to himself.